1. Field of the Invention
The present invention relates to an electronic appliance and particularly to an earphone microphone-mountable electronic appliance.
2. Description of the Related Art
To conventional electronic appliances such as mobile phones, earphone microphones provided with PTT (Push To Talk) buttons (hereinafter referred to as “PTT earphone microphones”) and earphone microphones without PTT buttons are mountable. The plugs of the PTT earphone microphones differ from those of the earphone microphones in that PTT button pins are provided, but the shapes of the plugs of the PTT earphone microphones with the PTT button pins excluded are the same as those of the plugs of the earphone microphones. In some cases, however, the plugs of the PTT earphone microphones differ from those of the earphone microphones in arrangement of the contact points. For example, a grounded portion and a portion for a right audio signal (R signal) input to the right earphone or for a left audio signal (L signal) input to the left earphone might be arranged in reserve.
This poses the problem of having to provide separate jacks for the plug of the PTT earphone microphone and that of the earphone microphone, thus necessitating a plurality of associated circuits.
In view of this, a single jack to which both the plugs of the PTT earphone microphone and the earphone microphone are mountable is contemplated. However, this necessitates judgment as to whether the plug of the PTT earphone microphone or that of the earphone microphone is being mounted.
A known method for judgment between a stereo headphone plug and an earphone microphone plug utilizes the fact that impedance varies in a large degree between the stereo headphone and the earphone microphone. However, this involves application of DC voltage to the stereo headphone; if not prepared for voltage application, the stereo headphone might be damaged, or fail in the worst case. Thus, it is impossible to use the conventional judgment method that involves detection of plug insertion while predetermining the location of the grounded portion in the plug.